Author Adam Sternbergh cites research from the '70s, which shows that the average mother would have more than three children in her lifetime.

"Back then, 40 percent of mothers between 40 and 44 had four or more children," explains Adam. "Twenty-five percent had three kids; 24 percent had two; and 11 percent had one."

Last year in the UK, figures from the Office for National Statistics showed the number of children women are likely to have during their childbearing years has dropped to its lowest level. Women aged 45 in 2016 were reported to have on average, 1.90 children (down from 2.21 for their parents' generation).

"The American Middle Child is now an endangered species," writes Adam. "As the ideal number of children per family has shrunk to two."

Last year, the average age of raising a child until their 18th birthday – excluding rent and childcare – has increased by almost four per cent in just one year, from £72,600 in 2016 to £75,400 in 2017.

Experts have also pointed out that millennials are more likely to delay childbirth.

“The average age of mums and dads in England and Wales has increased by almost 4 years over the last 4 decades," says statistician Nicola Haines.

"At the birth of a child in 2015, fathers averaged 33.2 years of age and mothers 30.3 years. Falling birth rates among the under-30s and rising birth rates at older ages reflect trends evident since the mid-1970s to delay childbearing to later ages."

Middle children all over the world have been quick to comment on the news. One Twitter user wrote: "The fact that middle children are going extinct, is the most middle child thing I've ever heard".

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